Dispersive acoustic line using two-layer fluid media



March 25, W69 P. TOURNOIS 3,435,331

DISPERSIVE ACOUSTIC LINE USING TWO-LAYER FLUID MEDIA Filed April 25, 1966 Sheet of 5 FiG.2

March 25, 1969 P. TOURNOIS 3,435,381

DISPERSIVE ACOUSTIC LINE USING TWO-LAYER FLUID MEDIA Filed April 25. 1966 Sheet 2 of 5 March 25, 1969 P. TOURNOIS 3,435,381

DISPERSIVE ACOUSTIC LINE USING TWO-LAYER FLUID MEDIA Filed April 25, 1966 Sheet 3 of 5 IIIIIJIIIIIIII'IIIIIII A March 1969 v P. TOURNOIS 3,435,381

DISPERSIVE ACOUSTIC LINE USING TWO-LAYER FLUID MEDIA Filed April 25, 1966 Sheet 4 of 5 F/Ge6 March 25, 1969 p TQURNQIS 3,435,381

DISPERSIVE ACOUSTIC LINE USING TWO-LAYER FLUID MEDIA Filed April 25, 1966 Sheet 5 of 5 .IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII x 15,518 Int. Cl. H03k 7/30; H01p 3/06 US. Cl. 33330 10 Claims There exist many types of dispersive lines for compressing pulses. They use as propagating medium strips, wires or stratified solids. Such dispersive lines are in particular used in radar systems, to increase the range whilst maintaining an excellent resolution.

The pulse compression technique is also applicable to sonar equipment; provided the dispersive lines are suitable for low frequencies. However, since the duration of the pulses to be compressed is then of an entirely different order of magnitude, lines of known types are generally unsuitable. Since the frequency excursion is reduced in this application and the pulses to be compressed have a substantially longer duration, the delays are insuflicient to produce effectivel the desired compression.

It is an object of the invention to provide a delay line which is suitable for compressing acoustic pulses.

According to the invention there is provided a dispersive acoustic line for compressing electrical signals frequency modulated on a carrier wave, comprising: a hollow acoustic waveguide having two ends; first and second fluid media filling said guide; said media forming a first and a second superimposed layer extending from one of said ends to the other; said first fluid medium propagating acoustic waves with a higher phase velocity than said second fluid medium; first electroacoustic transducer means coupled to said second layer at one of said ends for generating in said first and second media an acoustic compressional wave in response to said electrical signals and second electroacoustic transducer means coupled to said second layer at said other end for receiving said compressional wave.

For a better understanding of the invention and to show how the same may be carried into eflect reference will be made to the drawings accompanying the following description and in which:

FIG. 1 shows a stratified medium as used in a delay line according to the invention;

FIG. 2 is an explanatory drawing;

FIG. 3 is a chart, showing the properties of the stratified medium of FIG. 1;

FIG. 4 shows the wave trains propagating between the ends of a dispersive line;

FIG. 5 shows one embodiment of a dispersive line according to the invention;

FIG. 6 is a further embodiment of a dispersive line according to the invention; and

FIG. 7 is a partial view showing a coupling arrangement according to the invention.

As will be explained hereinafter in more detail, a dispersive line according to the invention comprises a stratified medium in which acoustic compression waves are propagated. For the sake of brevity and in order to distinguish them from other known waves, the waves propagating in such a medium will be hereinafter called D waves.

FIG. 1 shows a reference frame oxyz and a stratified medium portion comprising, from the bottom to the top, a first fluid medium 1, which is upwardly bounded by the plane zoz, a fluid layer 2 of constant thickness e, and a fiat and polished, rigid plate 3. In the liquid or gaseous nited States Patent 0 layers thus defined, scalar acoustic pressures are propagated as a function of the three coordinates and the time. Their expressions, the monochromatic solutions of the propagation equations, are given, using the conventional mathematical, notations by the following products of functions; in the layer 1:

The expression of D waves, capable of propagation within the stratified medium of FIG. 1, are obtained by considering the propagation along x, While ignoring the variations in the acoustic pressure along z; the above solutions have then the following forms: in layer 1:

10 3. e[ 1 in layer 2:

The condition of the existence of D Waves is given by the relation:

It follows that the D wave characterized by the wave number k is propagated along ox, provided its phase velocity C is linked with the respective phase velocities of the media 1 and 2 by the inequalities It may be shown that this inequality can be satisfied for a plurality of types of D waves, since there exists an infinity of possible transverse distributions of pressure amplitudes along 03 The D wave presents in fact an amplitude distribution which varies harmonically within the layer 2 and which extends into the layer 1 with an exponential decay. The acoustic pressure distribution at the abscissa x is propagated in the direction 0x with the phase velocity C.

FIG. 2 shows the depth distribution of acoustic pressures p for the D waves of the order 0 (a), order 1 (b) and order 2 (c). It can be seen that the acoustic pressure is at its maximum at the rigid wall 3 and decreases exponentially towards the base. If the thickness of medium 1 is defined by a base which is suificiently remote from the upper surface, no parasitic reflection can occur.

FIG. 3 summarizes graphically the properties of D waves transmitted through a stratified medium in accordance with the invention. By way of example, the curves have been plotted for a layer 2 of sulphur hexafluoride (SP and a layer 1 of air. The variable along the abscissa is the parameter ef/C where e is the thickness of the layer 2, f the frequency of the D wave and C the phase velocity characteristic of the fluid of the layer 2. The curves C/C and vg/C represent the frequency variations of the phase and group velocities of the Wave D. The curve t /r shows how the ratio of the group varies delay time t to the delay time 1' of a plane wave passing 3 through the layer 2 with the velocity C The drawing shows on the left the two sets of curves corresponding to the order and on the right to the mode order 1.

Physically, the D wave can be generated in the stratified medium by a plane acoustic Wave penetrating into the layer 2 at an incidence 0. For this reason, FIG. 3 shows in dotted line the angle 0 of incidence of the plane generating wave and of the zero order D wave. The value of this angle of incidence is derived from the relation The plane Wave generating the D wave undergoes alternating reflections on the rigid wall 3 and on the acoustic diopter formed by the flat junction between the two fluids. The D wave is the sum of all waves reflected and refracted in this stratified medium.

FIG. shows an embodiment of the dispersive line according to the invention. It comprises an acoustic wave guide of rectangular cross-section shaped as a parallelepipedal box 4 and a cover 3 equipped with oblique inlets 5 and 6. A separating diaphragm 7, which is both unpermeable and very thin, divides the guide into a first cavity 1, containing a first fluid, and a second cavity 2, containing a second fluid with a lower phase velocity than that of the first fluid. The diaphragm is an elastic film with a Youngs modulus of less than 20 kg./mm. By way of example, a film of polythene may be used with a thickness of 5-10 microns. The end faces of the wave guide are provided on the insides with nonreflecting layers 8 and 9. The inlets 5 and -6 are closed by electroacoustic transducers 10 and 11 of which only the radiating surface is represented by crosshatching. The front wall has 'been removed for the sake of clarity. The height of the vessel guide is so selected that the D wave is sufficiently damped on reaching the rigid base, and the lateral walls of the wave guide are spaced by a distance much greater than the thickness of the layer so as to minimize the disturbing action of the lateral walls (end effects).

In operation, an electric signal V such as that shown in FIG. 4, is applied to the electroacoustic transducer 10. This signal creates in the stratified medium a D wave propataging parallel to 0x and emerging from the layer at the end of the line, where it is recovered by the transducer 11 which supplies an electric signal V The suitable angle of incidence is obtained by means of the two inlet caps 5 and -6 which make possible the matching of the phase velocities between the transducers and the stratified medium.

The compressed signal V delivered by the dispersive line has for envelope the Fourier transform of the envelope of the input signal under the condition that the carrier of the input signal V is linearly modulated in frequency in the frequency range where the group delay time of the D wave varies linearly as a function of the frequency. FIG. 4 relates to the compression of a rectangular pulse with the duration T linearly modulated in frequency in a band A about the centre frequency f The compressed pulse has at midheight width a duration equal to the reciprocal of the modulation range A and its amplitude has been increased with respect to the input signal in the proportion x/TAf, which is the square root of the dispersion factor.

The dimensioning of the dispersive line according to the invention assumes that curves such as those of FIG. 3 are available for all the liquid or gaseous fluids which are used in pairs. These diagrams indicate the coefficient Q=Af/f which determines the range A) for which the group delay time t varies linearly as a function of the frequency. They also permit the determination of the maximum dispersion (At of the delay times t as a function of the frequency per unit of length of the dispersive line.

Starting from the desired duration of the compressed pulse, one calculates the frequency band M of the frequency modulation required. One selects then two suitable fluids and calculates the centre frequency i from the relation f =Af/Q. Since f and C are known, the

thickness e of the layer 2 can be obtained without difiiculty.

There remains only to determine the length of the line, it being known that this length x is such that the maximum delay At along the line must meet the following triple equality:

Thus, knowing T and (M the length of the line to be provided can be easily calculated.

By way of nonlimitative example the following table summarizes the essential data for four dispersive lines using gas and capable of solving in accordance with the invention the following problem: to compress a 20 ms. pulse into a pulse of 200 ,uS., which represents a disper- FIG. 6 shows a modification of the dispersion line according to the invention; it is formed by a rigid wave guide which is shaped as a spiral and consists of a vessel 4 and a cover 3. An impermeable diaphragm 7 separates the superimposed channels filled with the fluids 1 and 2. At the ends of the spiral are provided matched acoustic loads 8 and 9 as in the embodiment shown in FIG. 5 and inclined caps 5 and 6. The caps are equipped with electroacoustic transducers 10 and 1.1.

This embodiment of the dispersion line is less bulky due to its shape. By way of example, the transducers used in the case of gaseous fluids are electrodynarnic loudspeakers. The diaphragm separating the gases is a soft film having a high elasticity and negligible mass.

In FIG. 7, where the same references designate the same elements as in FIG. 5, there is shown a modification of the input caps serving to couple the transducers to superimposed fluids. According to the invention, each end of the line has two symmetrical branches forming a V-shaped arrangement whose channels 5 and 13 are inclined at an angle of incidence 0 on either side of the normal N to the diaphragm 7. The channel 5 is covered by the transducer 10 and the channel 13 terminates in an absorbent acoustic load 12. This structure differs from that of FIG. 5 in that the wave reflected by the diaphragm 7 under the incidence 0 is absorbed so that it cannot interfere with the operative of the line.

Of course, the invention is not limited to the embodiment described and shown which were given solely by way of example.

What is claimed is:

1. A dispersive acoustic line for compressing electrical signals, frequency modulated on a carrier wave, comprising: a hollow acoustic waveguide having two ends; first and second fluid media filling said guide; said media forming a first and a second superimposed layer extending from one of said ends to the other; said first fiuid medium propagating acoustic waves with a higher phase velocity than said second fluid medium; first electroacoustic transducer means coupled to said second layer at one of said ends for generating in said first and second media an acoustic compressional wave in response to said electrical signals and second electroacoustic transducer means coupled to said second layer at said other end for receiving said compressional wave.

2. A dispersive acoustic line as claimed in claim 1, further comprising an elastic diaphragm separating said media from each other.

3. A dispersive acoustic line as claimed in claim 1, wherein said guide comprises at each of said ends an inlet having an axis, said axis forming an angle with the normal to said second layer; the since of said angle being equal to the ratio of the phase velocity of the acoustic waves propagating through said second medium to the mean propagation velocity of said compressional wave; said inlets being filled with said second medium and said transducer means being respectively coupled to said inlets.

4. A dispersive acoustic line as claimed in claim 3, further comprising an outlet at each of said ends; said outlet having an axis symmetrical of said axis with respect to said normal and absorbing means coupled to said outlet for absorbing the acoustic wave reflected on said second layer.

5. A dispersive acoustic line as claimed in claim 1, wherein said transducer means respectively comprise dynamic loudspeakers respectively exciting and receiving said compressional wave near the respective ends of said waveguide, and means for absorbing said compressional wave being provided at said ends.

6. A dispersive acoustic line as claimed in claim 1, wherein said fluid media are liquids.

References Cited UNITED STATES PATENTS 3,353,120 11/1967 Tournois 333 ROY LAKE, Primary Examiner.

DARWIN R. HOSTETTER, Assistant Examiner.

US. Cl. X.R. 

1. A DISPERSIVE ACOUSTIC LINE FOR COMPRESSING ELECTRICAL SIGNALS, FREQUENCY MODULATED ON A CARRIER WAVE, COMPRISING; A HOLLOW ACOUSTIC WAVEGUIDE HAVING TWO ENDS; FIRST AND SECOND FLUID MEDIA FILLING SAID GUIDE; SAID MEDIA FORMING A FIRST AND A SECOND SUPERIMPOSED LAYER EXTENDING FROM ONE OF SAID ENDS TO THE OTHER; SAID FIRST FLUID MEDIUM PROPAGATING ACOUSTIC WAVES WITH A HIGHER PHASE VELOCITY THAN SAID SECOND FLUID MEDIUM; FIRST ELECTROACOUSTIC TRANSDUCER MEANS COUPLED TO SAID SECOND LAYER AT ONE OF SAID ENDS FOR GENERATING IN SAID FIRST AND SECOND MEDIA AN ACOUSTIC COMPRESSIONAL WAVE IN RESPONSE TO SAID ELECTRICAL SIGNALS AND SECOND ELECTROACOUSTIC TRANSDUCER MEANS COUPLED TO SAID SECOND LAYER AT SAID OTHER END FOR RECEIVING SAID COMPRESSIONAL WAVE. 